1. Field of Invention
The present invention relates in general to electrical well logging, and pertains in particular to improved method and apparatus for simultaneously investigating underground formation zones which are near and far from a borehole that traverses the formation zones, which method and apparatus are compatible for use along with induction logging method and apparatus.
2. The Prior Art
It has been the practice to investigate earth formations surrounding a borehole by lowering a sonde in the borehole so as to measure characteristics of the traversed formation such as conductivity, resistivity, porosity, etc. The formation conductivity is mainly measured by generating in the formation coil-induced electromagnetic currents and thereafter detecting with receiving coils magnetic fields generated by the induced currents. Such an apparatus is commonly called an induction sonde and the recording of the measurements versus depth is called an induction log. Reference may be had to U.S. Pat. No. 2,582,314 granted to Henri G. Doll on Jan. 15, 1952, and assigned to the Schlumberger Technology Corp. for a description of such an apparatus.
Likewise it is known that formation resistivity measurements are obtained by passing survey electrical currents through the formation via electrodes and measuring voltages between certain of the electrodes. In order to minimize the influence of the borehole and those portions of the formation which are close to the wall of the borehole, the survey current is focused into the formations by means of special electrodes to provide a mode of operation known as deep investigation as opposed to unfocused shallow investigation. Such a focusing electrode logging tool has been described in U.S. Pat. No. 2,712,627 granted to H. G. Doll on July 5, 1955 and assigned to the Schlumberger Technology Corp.
It is well known that there are domains of resistivity in which induction measurements are better suited than resistivity measurements and vice versa. For example, when the ratio R.sub.t /R.sub.m is much greater than 1, such as in salty muds and/or in highly resistive formations, resistivity measurements are preferred; R.sub.m commonly designates the resistivity of drilling mud in the borehole, R.sub.xo the resistivity of the formation zone directly adjacent to the borehole and invaded by mud filtrate, and R.sub.t the resistivity of the uninvaded formation zone. On the contrary, when R.sub.t is smaller than R.sub.m or R.sub.xo, induction measurements are more satisfactory. Therefore, it has been suggested to combine on one sonde the focusing-electrode system of a resistivity sonde with a coil system of a conductivity or induction sonde. Such a system has been described in U.S. Pat. No. 3,124,742 granted to William P. Schneider on Mar. 10, 1964 and assigned to the Schlumberger Technology Corp. This patent describes a "resistivity-induction tool" allowing for the simultaneous recording of a resistivity log and an induction log on the same trip of the sonde through a well bore.
The main difficulty encountered in mounting together, in an interlaced manner, an electrode system and a coil system, arises from the sensitivity of the responses of the coils to the presence of conductive electrodes in close proximity therewith. Consequently, the number of electrodes of the sonde described in the Schneider patent, although fairly large, is minimized, and electrodes having large surfaces have been avoided. More precisely, the focusing-electrode system of the Schneider patent comprises a central current electrode A.sub.o, two pairs of monitoring electrodes M.sub.1 -M'.sub.1 and M.sub.2 -M'.sub.2, one pair of auxiliary current electrodes A.sub.1 -A'.sub.1 and one current return electrode B located on the sonde. The depth of investigation of such a system is shallow, due on the one hand to the small surface size and short spacing of the electrodes, and on the other hand to the location of the current return electrode B on the sonde. This arrangement presents a serious drawback since only a shallow investigation is possible whereas a deep investigation is often additionally required, especially when R.sub.t and R.sub.xo are substantially different.
Relatively deep and shallow depths of investigation can be simultaneously obtained with a known type of dual focusing-electrode system, called "Dual resistivity" and described in U.S. Pat. No. 2,712,630 granted to Henri G. Doll on July 5, 1955, and in U.S. Pat. No. 3,772,589 granted to Andre Scholberg on Nov. 13, 1973 (the two patents are assigned to the Schlumberger Technology Corp.). The dual resistivity system operates at two different frequencies f.sub.1 and f.sub.2 and comprises two current return electrodes, one on the sonde for the shallow investigation and one at the surface for the deep investigation. Besides, as shown in the Scholberg patent, the current return electrodes of the shallow investigation electrode system A.sub.2 -A'.sub.2, are used as auxiliary current electrodes for the deep investigation electrode system. These electrodes, however, are of a large size, in order to increase the depth of investigation, and therefore cannot be used in a combined dual resistivity-induction sonde without introducing large adverse effects into the operation of the coil system.
Additionally, it is the current practice to record along with the resistivity measurements the difference between the potential of an electrode in the borehole and a fixed potential of a surface electrode, commonly known as "spontaneous potential". Large size metallic electrodes employed for the deep investigation give rise to erroneous measurements of the spontaneous potential.